While signals within telecommunications and data communications networks have been traditionally exchanged by transmitting electrical signals via electrically conductive lines, an alternative medium of data exchange is the transmission of optical signals through optical fibers. Information is exchanged in the form of modulations of laser-produced light. The equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the design of optical switches for use in telecommunications and data communications networks is problematic. As a result, switching requirements within a network that transmits optical signals is often satisfied by converting the optical signals to electrical signals at the inputs of a switching network, then reconverting the electrical signals to optical signals at the outputs of the switching network.
Recently, reliable optical switching systems have been developed. U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers. Another such matrix of switching elements is described in U.S. Pat. No. 4,988,157 to Jackel et al. An isolated switching element 10 is shown in FIG. 1, while a 5.times.5 matrix 32 of switching elements is shown in FIG. 2. The optical switch of FIG. 1 is formed on a substrate. The substrate may be a silicon substrate, but other materials may be used. The optical switch 10 includes planar waveguides defined by a lower cladding layer 14, a core 16, and an upper cladding layer, not shown. The core is primarily silicon dioxide, but with other materials that achieve a desired index of refraction for the core. The cladding layers are formed of a material having a refractive index that is different from the refractive index of the core material, so that optical signals are guided along the waveguides.
The core material 16 is patterned to form an input waveguide 20 and an output waveguide 26 of a first optical path and to define a second input waveguide 24 and a second output waveguide 22 of a second optical path. The upper cladding layer is then deposited over the patterned core material. A gap is formed by etching a trench 28 through the core material and the two cladding layers to the substrate. The waveguides intersect the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the crosspoint 30 of the waveguides is filled with a vapor or gas. Thus, TIR diverts light from the input waveguide 20 to the output waveguide 22, unless an index-matching fluid resides within the crosspoint 30 between the aligned waveguides 20 and 26. The trench 28 is positioned with respect to the four waveguides such that one sidewall of the trench passes through or slightly offset from the intersection of the axes of the waveguides.
The above-identified patent to Fouquet et al. describes a number of alternative approaches to switching the switching element 10 between a transmissive state and a reflective state. The element includes at least one heater that can be used to manipulate fluid within the trench 28. One approach is illustrated in FIG. 1. The switching element 10 includes two microheaters 38 and 40 that control the position of a bubble within the fluid-containing trench. The fluid within the trench has a refractive index that is close to the refractive index of the core material 16 of the four waveguides 20-26. Fluid fill-holes 34 and 36 may be used to provide a steady supply of fluid, but this is not critical. In the operation of the switching element, one of the heaters 38 and 40 is brought to a temperature sufficiently high to form a gas bubble. Once formed, the bubble can be maintained in position with a reduced current to the heater. In FIG. 1, the bubble is positioned at the crosspoint 30 of the four waveguides. Consequently, an input signal along the waveguide 20 will encounter a refractive index mismatch upon reaching the trench 28. This places the switching element in a reflecting state, causing the optical signal along the waveguide 20 to be redirected to the output waveguide 22. However, even in the reflecting state, the second input waveguide 24 is not in communication with the output waveguide 26.
If the heater 38 at crosspoint 30 is deactivated and the second heater 40 is activated, the bubble will be attracted to the off-axis heater 40. This allows index-matching fluid to fill the crosspoint 30 of the waveguides 20-26. The switching element 10 is then in a transmitting state, since the input waveguide 20 is optically coupled to the collinear waveguide 26.
In the 5.times.5 matrix 32 of FIG. 2, any of the five input waveguides 42, 44, 46, 48 and 50 may be optically coupled to any one of the five output waveguides 52, 54, 56, 58 and 60. The switching matrix is sometimes referred to as a "non-blocking" matrix, since any free input fiber can be connected to any free output fiber regardless of which connections have already been made through the switching matrix. Each of the twenty-five optical switches has a trench that causes TIR in the absence of a fluid at the crosspoint of the waveguides, but two collinear waveguides of a particular waveguide path are optically coupled when the crosspoint associated with the waveguides is filled with the fluid. Trenches that are in the transmissive state are represented by fine lines that extend at an angle through the intersections of the optical waveguides in the matrix. On the other hand, trenches of switching elements in a reflecting state are represented by broad lines through points of intersection.
In FIGS. 1 and 2, the input waveguide 20 is in optical communication with the output waveguide 22, as a result of TIR at the crosspoint 30. Since all other crosspoints for allowing the input waveguide 48 to communicate with the output waveguide 54 are in a transmissive state, a signal that is generated at input waveguide 48 will be received at output waveguide 54. In like manner, the input waveguide 42 is optically coupled to the output waveguide 60, the input waveguide 44 is optically coupled to the output waveguide 56, the input waveguide 46 is optically coupled to the output waveguide 52, and the input waveguide 50 is optically coupled to the output waveguide 58.
One concern with optical switching elements 10 of this type is that in the transmissive state, there is a small but potentially objectionable amount of reflection. If the index of refraction of the fluid is different than that of the core material 16, reflections occur. A precise match between the indices of refraction is problematic, since there are other considerations in the selection of a fluid. For example, since the fluid is manipulated using thermal energy, the thermal properties of the liquid must be considered. Thus, there is a loss of signal strength at each transmissive crosspoint within the matrix 32.
In FIG. 2, an optical signal that enters the input waveguide 48 Input Port 3) will pass through one crosspoint in the horizontal direction and three crosspoints in the vertical direction, since the optical signal will be reflected at the switching element 10. The total loss is 1 k+3 k=4 k, where k is the loss associated with each crosspoint. On the other hand, an optical signal entering the input waveguide 50 (Input Port 4) will propagate through the output waveguide 58 (Output Port 3), thereby passing through three crosspoints in the horizontal direction and four crosspoints in the vertical direction. The total loss is 3 k+4 k=7 k. In the figure, the input ports are shown as having a ranking from .0. to 4. Likewise, the output ports are shown as having a ranking from .0. to 4. The lower order inputs require fewer traversals of the optical crosspoints for coupling to a given output, while the lower order outputs require fewer crosspoint traversals by an optical signal for coupling to a given input. Thus, there are location-dependent losses within the operation of the matrix. There is also a loss that is encountered at each crosspoint that is in the reflecting state, but this loss is not dependent upon location.
Concerns with location-dependent losses are compounded when switching units such as the one found in FIG. 2 are cascaded to form a multistage switch. A multistage switch formed of cascaded switching units is described in U.S. Pat. No. 5,903,686 to MacDonald. The switch enables manipulation of P optical signals to P locations in a non-blocking manner using a number of interconnected functionally identical switching units. The multistage switch is intended to be an improvement over the conventional "Clos" network. According to the Clos design, a three-stage switch includes P input switching units having N input ports and M output ports. The conventional Clos switch also includes M middle stage switching units of P input ports and P output ports. Finally, there are P output switching units of M input ports and N output ports. For such a switch to be non-blocking without requiring additional connections, M=2N-1. It has been posited that with intelligent routing, the minimum value of M is 3/2N, but this has not been proven.
In following the conventional Clos design, the least loss path through the switch is .0.k and the greatest loss path is 2(N+M+P-3)k. FIG. 3 illustrates a conventional interconnect technique for a three-stage switch. In the switch of FIG. 3, there are five input switching units and five output switching units (P=5), but only three input switching units and three output switching units are shown. In the figure, M=5 and N=5. Moreover, there are five middle stage switching units M.sub.2 =5. For clarity, only some of the interconnections are shown in the figure. The switching units are interconnected by coupling output port .0. at input unit 1 to input port .0. of middle unit 1, output port 1 of input unit 1 to input port .0. of middle unit 2, and so. The output port .0. of input unit 2 is connected to the middle unit input port 1, output port 1 is connected to middle unit 1 input port 1, and so on. The switch structure is symmetrical, so that connections from the middle units to the output units follow the same pattern.
Although the minimum number of middle stage switching units is 2N-1, there is an advantage to using 2 m middle units, since this provides non-blocking routing even with single crosspoint failures in the input and output switching units or with the failure of a complete middle unit.
A larger scale switch may be a 128.times.128 three-stage non-blocking crosspoint switch with fault tolerance. In such a switch, there would be sixteen input switching units (i.e., P=16) of size 8.times.16 (i.e., N=8, M=16), sixteen middle units (i.e., M.sub.2 =16) of size 16.times.16, and sixteen output switching units of size 16.times.8. With an interconnect as described with reference to FIG. 3, the least loss would be .0.k and the worst loss would be 74 k, or 37 k.+-.37 k. This worst case scenario of 74k would accumulate from a total of 22 k in the input stage, 3.0.k in the middle stage, and 22k in the output stage.
What is needed is a multistage optical switch having an increased level of uniformity with respect to location-dependent losses.